With growing concern about the quantity of sulfur oxides, particularly sulfur dioxide, being introduced into the atmosphere from industrial stack gases, emphasis has focused upon developing an air pollution control system that is capable of achieving a high degree of sulfur dioxide removal from such industrial stack gases. This objective may generally be accomplished by providing a scrubber system wherein an excess of sulfur dioxide reactant material is provided for reaction. However, this approach has two serious disadvantages which offset the advantage of sulfur dioxide removal. The first disadvantage is the additional costs of the excess supply of sulfur dioxide reactant material. The second is the large quantities of solid waste material discharged from such a system. Such waste material must be disposed of in a manner consistent with current antipollution codes. Further, in such a system the pH of liquids therein, particularly within the scrubbing or contact zone of the scrubber, may be effectively changed by changes in the sulfur content of the fuel, or by changes in the ratio of additive to fuel, or even by changes in the reactivity of the sulfur dioxide reactant material. It is generally found that when the pH in the contact zone is too low the scrubber itself is subject to acid attack. Alternatively, when the pH is too high precipitation occurs in the contact zone and causes scaling which reduces the efficiency of the overall system. Further, it has been observed that to obtain optimum scrubbing of an industrial stack gas in a scrubber, the gas flow velocity therethrough should be maintained within the range of several hundred feed per minute. To meet this requirement air pollution control systems have been designed which have multiple scrubbers operated in parallel and which are selectively removed or inserted into the stream as the gas load varies. In this manner, the industrial stack gas flow rate through a given scrubber is maintained in the proper range. The use of a plurality of scrubbers operated in parallel however, serves to compound the problem of optimizing and controlling the sulfur dioxide reactant material concentrations and the pH within the various scrubbers. In addition, because the industrial stack gas paths from the source to the several scrubbers in parallel may vary considerably from one to another in length and geometry, the demands for sulfur dioxide reactant material in each of the scrubbers may differ significantly from one to another, thereby resulting in different sulfur dioxide removal conditions at each scrubber. Such variations may result in comparable variations in the pH's within the contact zone of the scrubbers. The present invention overcomes the above described prior art problems and provides a straightforward, inexpensive, readily employed and easily operated system for removal of sulfur dioxide from industrial stack gases eliminating much of the equipment previously required using a minimum of space and, at the same time, providing effective control.